The present invention relates to foam-in-place packaging and, more particularly, to an improved sealing device for making foam-in-place packaging cushions.
Foam-in-place packaging is a highly useful technique for on demand protection of packaged objects. In its most basic form, foam-in-place packaging comprises injecting foamable compositions from a dispenser into a container that holds an object to be cushioned. Typically, the object is wrapped in plastic to keep it from direct contact with the rising (expanding) foam. As the foam rises, it expands into the remaining space between the object and its container (e.g. a corrugated board box) thus forming a custom cushion for the object.
A common foamable composition is formed by mixing an isocyanate compound with a hydroxyl-containing material, such as a polyol (i.e., a compound that contains multiple hydroxyl groups), typically in the presence of water and a catalyst. The isocyanate and polyol precursors react to form polyurethane. At the same time, the water reacts with the isocyanate compound to produce carbon dioxide. The carbon dioxide causes the polyurethane to expand into a foamed cellular structure, i.e., a polyurethane foam, which serves to protect the packaged object.
In other types of foam-in-place packaging, an automated apparatus produces flexible bags from flexible, plastic film and dispenses a foamable composition into the bags as the bags are being formed. As the composition expands into a foam within the bag, the bag is sealed shut and typically dropped into a container holding the object to be cushioned. The rising foam again tends to expand into the available space, but does so inside the bag. Because the bags are formed of flexible plastic, they form individual custom foam cushions around the packaged objects. Exemplary types of such packaging apparatus are assigned to the assignee of the present invention, and are illustrated, for example, in U.S. Pat. Nos. 4,800,708, 4,854,109, 5,027,583, 5,376,219, and 6,003,288, the contents of each of which are incorporated entirely herein by reference.
As will be noted from the foregoing patents, the typical method of forming a foam-in-place cushion from a plastic bag is to heat-seal plies of plastic film material together both transversely and longitudinally as they are being fed from a stock supply to form a generally rectangular bag with the foam inside. In alternative embodiments, the stock supply of plastic film material can be fed in center-folded fashion so that one side of the bag is a fold, rather than a heat seal. Foam-in-place packaging apparatus that operate in accordance with such methods have gained rapid and wide acceptance in the marketplace and have served their purposes well.
However, improvement in the operation of such apparatus is continually sought. One aspect in particular where improvement is desired concerns the manner in which the film plies are sealed together, especially in the longitudinal direction, i.e., the direction in which the film plies move as they are conveyed through the packaging apparatus. Conventional devices for making longitudinal heat seals generally include a sealing wire or other electrical heating means wrapped around and affixed to the circumference of a rotatable cylinder such that the wire rotates along with the cylinder. To effect a heat seal, the entire wire is heated, by sending an electrical current therethrough, and the cylinder is brought into rotational contact with the film plies to be sealed as the film plies move against and past the heated cylinder. While this approach has generally been effective, its main drawback is that consistent sealing temperatures are difficult to achieve.
The inventors hereof have determined that the cause of such inconsistent sealing temperatures is due to two primary factors: first, the entire wire is heated and, second, the wire is wrapped about the entire circumference of the rotatable cylinder. As a number of foam-in-place cushions are made in succession during continuous operation of the packaging apparatus, this configuration results in a gradual but steady increase in the temperature of the cylinder because the cylinder continually absorbs heat from the wire at a faster rate than the rate at which the heat can be dissipated to the air. Without careful control and monitoring of the longitudinal heat-seal device, the ever-changing temperature of the rotating cylinder often results in inconsistent seals being made between the film plies that form the flexible container for the expanding foam. As a consequence, insufficient heat necessary to form complete (strong) heat-seals may be applied at the beginning of a cushion-making session, which can cause the seal to be forced open as the foam expands within the cushion, and/or an excessive amount of heat may be applied toward the end of the session, which causes the cylinder to melt through the film plies and produce holes in the cushion. When the heat-seal device melts through the film plies, an outer strip form one or both film plies very often separates from the rest of the film and wraps around the longitudinal heat-seal device. This results in the necessity of shutting-down the cushion-making apparatus and extricating the film strip from the heat-seal device. Typically, the strip is tightly wound around the device and/or partially melted such that removal of the strip is a difficult and time-consuming process.
Another drawback of the xe2x80x98wrap-aroundxe2x80x99 heat-seal device as discussed above is that routine maintenance procedures therefor are more cumbersome and time-consuming than would otherwise be desired. In particular, the sealing wire is difficult to remove and replace when worn.
Finally, since the entire wire must be heated, the energy requirements for the sealing device are higher than desired.
Therefore, the need exists for an improved heat-seal device that is suitable for forming longitudinal heat-seals for foam-in-place packaging cushions.
That need is met by the present invention, which, in one aspect, provides a device for sealing two plies of plastic film together, comprising:
a. an electrically conductive heating element having a first end secured to a first node and a second end secured to a second node; and
b. a rotatable support cylinder having an outer, circumferential surface, the cylinder being positioned in abutting relationship with the heating element and capable of rotating against the heating element along a predetermined contact arc such that less than a complete circumference of the outer surface of the cylinder is in contact with the heating element,
whereby, two plies of juxtaposed plastic film may be sealed together by bringing the juxtaposed film plies into contact with the heating element along the contact arc and causing sufficient current to flow through the heating element that the heating element heats to a sealing temperature that causes the juxtaposed film plies to seal together.
Another aspect of the present invention is an apparatus for dispensing fluid into flexible containers and enclosing the fluid within the containers, comprising:
a. a mechanism that conveys a web of film along a predetermined path of travel, the film web comprising two juxtaposed plies of plastic film that define a partially-formed flexible container;
b. a dispenser through which fluid may flow in predetermined amounts, said dispenser positioned adjacent the travel path of the film web such that the dispenser can dispense the fluid into the partially-formed flexible container; and
c. a device for sealing the plies of plastic film together so that the partially-formed container can be completed to thereby enclose the fluid therein, with the contact arc of the sealing device being positioned in the travel path of the film web,
whereby, the two plies of juxtaposed plastic film are sealed together when the conveying mechanism brings the film web into moving contact with the heating element along the contact arc and sufficient current is caused to flow through the heating element that the heating element heats to a sealing temperature that causes the juxtaposed film plies to seal together.
A further aspect of the present invention is a system for dispensing fluid into flexible containers and enclosing the fluid within the containers, comprising:
a. a web of film comprising two juxtaposed plies of plastic film that define a partially-formed flexible container;
b. a mechanism that conveys the film web along a predetermined path of travel;
c. a fluid source containing therein a fluid;
d. a dispenser through which fluid may flow in predetermined amounts, the dispenser being in fluid communication with the fluid source and positioned adjacent the travel path of the film web such that the dispenser can dispense the fluid into the container; and
e. a device as described above for sealing the plies of plastic film together.
These and other aspects and features of the invention may be better understood with reference to the following description and accompanying drawings.